I'm sorry the school district is being so difficult. Make sure to get all their refusals in a PWN. Another thought is to ask for a facilitated IEP meeting. See if your state provides those. Part of the state facilitator's job is to make sure all parties participate.
If nothing works, you could file a state complaint setting forth the school district's lack of compliance in allowing meaningful parent participation. See if there if a state plan or policy on the state department of education's website that you could quote.